Dust collector

A dust collector is a system used to enhance the quality of air released from industrial and
commercial processes by collecting dust and other impurities from air or gas. Designed to
handle high-volume dust loads, a dust collector system consists of a blower, dust filter, a filter-
cleaning system, and a dust receptacle or dust removal system. It is distinguished from air
cleaners, which use disposable filters to remove dust
Dust collectors are used in many processes to either recover valuable solid or powder from
process streams, or to remove granular solid pollutants from exhaust gases prior to venting to
the atmosphere. Dust collection is an online process for collecting any process-generated dust
from the source point on a continuous basis. Dust collectors may be of single unit construction,
or a collection of devices used to separate particulate matter from the process air. They are
often used as an air pollution control device to maintain or improve air quality.

Mist collectors remove particulate matter in the form of fine liquid droplets from the air. They
are often used for the collection of metal working fluids, and coolant or oil mists. Mist collectors
are often used to improve or maintain the quality of air in the workplace environment.

Fume and smoke collectors are used to remove sub-micrometer-size particulates from the air.
They effectively reduce or eliminate particulate matter and gas streams from many industrial
processes such as welding, rubber and plastic processing, high speed machining with coolants,
tempering, and quenching.

Types of dust collectors

Five main types of industrial dust collectors are:

Inertial separators

Fabric filters

Wet scrubbers

Unit collectors

Electrostatic precipitators

Inertial separators separate dust from gas streams using a combination of forces, such as
centrifugal, gravitational, and inertial. These forces move the dust to an area where the forces
exerted by the gas stream are minimal. The separated dust is moved by gravity into a hopper,
where it is temporarily stored.

The three primary types of inertial separators are:

Settling chambers
Baffle chambers

Centrifugal collectors

Neither settling chambers nor baffle chambers are commonly used in the minerals processing
industry. However, their principles of operation are often incorporated into the design of more
efficient dust collectors.

Settling chamber

A settling chamber consists of a large box installed in the ductwork. The increase of cross
section area at the chamber reduces the speed of the dust-filled airstream and heavier particles
settle out. Settling chambers are simple in design and can be manufactured from almost any
material. However, they are seldom used as primary dust collectors because of their large space
requirements and low efficiency. A practical use is as precleaners for more efficient collect.

Advantages:
1>simple construction and low cost.

2>maintaince free.

3>collection of particle without need of water.

Disadvantages:

1>low efficiency.

2>large space required.

Baffle chamber

Baffle chambers use a fixed baffle plate that causes the conveying gas stream to make a sudden
change of direction. Large-diameter particles do not follow the gas stream but continue into a
dead air space and settle. Baffle chambers are used as precleaners

Centrifugal collectors

Centrifugal collectors use cyclonic action to separate dust particles from the gas stream. In a
typical cyclone, the dust gas stream enters at an angle and is spun rapidly. The centrifugal force
created by the circular flow throws the dust particles toward the wall of the cyclone. After
striking the wall, these particles fall into a hopper located underneath.

The most common types of centrifugal, or inertial, collectors in use today are:-

1)Single-cyclone separators

They create a dual vortex to separate coarse from fine dust. The main vortex spirals downward
and carries most of the coarser dust particles. The inner vortex, created near the bottom of the
cyclone, spirals upward and carries finer dust particles.

2)Multiple-cyclone separators
Multiple-cyclone separators consist of a number of small-diameter cyclones, operating in
parallel and having a common gas inlet and outlet, as shown in the figure, and operate on the
same principle as single cyclone separatorscreating an outer downward vortex and an
ascending inner vortex.

Multiple-cyclone separators remove more dust than single cyclone separators because the
individual cyclones have a greater length and smaller diameter. The longer length provides
longer residence time while the smaller diameter creates greater centrifugal force. These two
factors result in better separation of dust particulates. The pressure drop of multiple-cyclone
separators collectors is higher than that of single-cyclone separators, requiring more energy to
clean the same amount of air. A single-chamber cyclone separator of the same volume is more
economical, but doesn't remove as much dust.
Cyclone separators are found in all types of power and industrial applications, including pulp
and paper plants, cement plants, steel mills, petroleum coke plants, metallurgical plants, saw
mills and other kinds of facilities that process dust.

Secondary-air-flow separators

This type of cyclone uses a secondary air flow, injected into the cyclone to accomplish several
things. The secondary air flow increases the speed of the cyclonic action making the separator
more efficient; it intercepts the particulate before it reaches the interior walls of the unit; and it
forces the separated particulate toward the collection area. The secondary air flow protects the
separator from particulate abrasion and allows the separator to be installed horizontally
because gravity is not depended upon to move the separated particulate downward.

Fabric filters

Commonly known as baghouses, fabric collectors use filtration to separate dust particulates
from dusty gases. They are one of the most efficient and cost effective types of dust collectors
available and can achieve a collection efficiency of more than 99% for very fine particulates.[6]

Dust-laden gases enter the baghouse and pass through fabric bags that act as filters. The bags
can be of woven or felted cotton, synthetic, or glass-fiber material in either a tube or envelope
shape.
Pre-coating

To ensure the filter bags have a long usage life they are commonly coated with a filter enhancer
(pre-coat). The use of chemically inert limestone (calcium carbonate) is most common as it
maximises efficiency of dust collection (including fly ash) via formation of what is called a
dustcake or coating on the surface of the filter media. This not only traps fine particulates but
also provides protection for the bag itself from moisture, and oily or sticky particulates which
can bind the filter media. Without a pre-coat the filter bag allows fine particulates to bleed
through the bag filter system, especially during start-up, as the bag can only do part of the
filtration leaving the finer parts to the filter enhancer dustcake.[7][8]

Parts

Fabric filters generally have the following parts:

Clean plenum

Dusty plenum

Bag, cage, venturi assembly

Tubeplate

RAV/SCREW

Compressed air header

Blow pipe

Housing and hopper

Types of bag cleaning Edit

Baghouses are characterized by their cleaning method.[9]

Shaking

A rod connecting to the bag is powered by a motor. This provides motion to remove caked-on
particles. The speed and motion of the shaking depends on the design of the bag and
composition of the particulate matter. Generally shaking is horizontal. The top of the bag is
closed and the bottom is open. When shaken, the dust collected on the inside of the bag is
freed. During the cleaning process, no dirty gas flows through a bag while the bag is being
cleaned. This redirection of air flow illustrates why baghouses must be compartmentalized.

Reverse air

Air flow gives the bag structure. Dirty air flows through the bag from the inside, allowing dust to
collect on the interior surface. During cleaning, gas flow is restricted from a specific
compartment. Without the flowing air, the bags relax. The cylindrical bag contains rings that
prevent it from completely collapsing under the pressure of the air. A fan blows clean air in the
reverse direction. The relaxation and reverse air flow cause the dust cake to crumble and
release into the hopper. Upon the completion of the cleaning process, dirty air flow continues
and the bag regains its shape.

Pulse jet

This type of baghouse cleaning (also known as pressure-jet cleaning) is the most common. A
high pressure blast of air is used to remove dust from the bag. The blast enters the top of the
bag tube, temporarily ceasing the flow of dirty air. The shock of air causes a wave of expansion
to travel down the fabric. The flexing of the bag shatters and discharges the dust cake. The air
burst is about 0.1 second and it takes about 0.5 seconds for the shock wave to travel down the
length of the bag. Due to its rapid release, the blast of air does not interfere with contaminated
gas flow. Therefore, pulse-jet baghouses can operate continuously and are not usually
compartmentalized. The blast of compressed air must be powerful enough to ensure that the
shock wave will travel the entire length of the bag and fracture the dust cake. The efficiency of
the cleaning system allows to have much higher gas to cloth ratio (or volumetric throughput of
gas per unit area of filter) than shaking and reverse air bag filters[10]. This kind of filter thus
require smaller area to admit a same volume of air.
Sonic

The least common type of cleaning method is sonic. Shaking is achieved by sonic vibration. A
sound generator produces a low frequency sound that causes the bags to

Dust collectors that use liquid are known as wet scrubbers. In these systems, the scrubbing
liquid (usually water) comes into contact with a gas stream containing dust particles. Greater
contact of the gas and liquid streams yields higher dust removal efficiency.

There is a large variety of wet scrubbers; however, all have one of three basic configurations:

1. Gas-humidification - The gas-humidification process agglomerates fine particles, increasing

the bulk, making collection easier.

2. Gas-liquid contact - This is one of the most important factors affecting collection efficiency.
The particle and droplet come into contact by four primary mechanisms:

a) Inertial impaction - When water droplets placed in the path of a dust-laden gas stream, the
stream separates and flows around them. Due to inertia, the larger dust particles will continue
on in a straight path, hit the droplets, and become encapsulated.

b) Interception - Finer particles moving within a gas stream do not hit droplets directly but
brush against and adhere to them.

c) Diffusion - When liquid droplets are scattered among dust particles, the particles are
deposited on the droplet surfaces by Brownian movement, or diffusion. This is the principal
mechanism in the collection of submicrometre dust particles.

d) Condensation nucleation - If a gas passing through a scrubber is cooled below the dewpoint,
condensation of moisture occurs on the dust particles. This increase in particle size makes
collection easier.

3. Gas-liquid separation - Regardless of the contact mechanism used, as much liquid and dust as
possible must be removed. Once contact is made, dust particulates and water droplets combine
to form agglomerates. As the agglomerates grow larger, they settle into a collector.
The "cleaned" gases are normally passed through a mist eliminator (demister pads) to remove
water droplets from the gas stream. The dirty water from the scrubber system is either cleaned
and discharged or recycled to the scrubber. Dust is removed from the scrubber in a clarification
unit or a drag chain tank. In both systems solid material settles on the bottom of the tank. A
drag chain system removes the sludge and deposits in into a dumpster or stockpile.

Types of scrubbers

Spray-tower scrubber wet scrubbers may be categorized by pressure drop as follows:

Low-energy scrubbers (0.5 to 2.5 inches water gauge - 124.4 to 621.9 Pa)

Low- to medium-energy scrubbers (2.5 to 6 inches water gauge - 0.622 to 1.493 kPa)

Medium- to high-energy scrubbers (6 to 15 inches water gauge - 1.493 to 3.731 kPa)

High-energy scrubbers (greater than 15 inches water gauge - greater than 3.731 kPa)

Due to the large number of commercial scrubbers available, it is not possible to describe each
individual type here. However, the following sections provide examples of typical scrubbers in
each category.

Low-energy scrubbers

In the simple, gravity-spray-tower scrubber, liquid droplets formed by liquid atomized in spray
nozzles fall through rising exhaust gases. Dirty water is drained at the bottom.

These scrubbers operated at pressure drops of 1 to 2 in. water gauge ( to kPa) and are
approximately 70% efficient on 10 m particles. Their efficiency is poor below 10 m. However,
they are capable of treating relatively high dust concentrations without becoming plugged.

Low- to medium-energy scrubbers

Wet cyclones use centrifugal force to spin the dust particles (similar to a cyclone), and throw the
particulates upon the collector's wetted walls. Water introduced from the top to wet the
cyclone walls carries these particles away. The wetted walls also prevent dust reentrainment.

Pressure drops for these collectors range from 2 to 8 in. water ( to 2 kPa), and the collection
efficiency is good for 5 m particles and above.

high-energy scrubbers co-current-flow scrubber

Packed-bed scrubbers consist of beds of packing elements, such as coke, broken rock, rings,
saddles, or other manufactured

Electrostatic precipitators (ESP)

Electrostatic precipitators use electrostatic forces to separate dust particles from exhaust gases.
A number of high-voltage, direct-current discharge electrodes are placed between grounded
collecting electrodes. The contaminated gases flow through the passage formed by the
discharge and collecting electrodes. Electrostatic precipitators operate on the same principle as
home "Ionic" air purifiers.

The airborne particles receive a negative charge as they pass through the ionized field between
the electrodes. These charged particles are then attracted to a grounded or positively charged
electrode and adhere to it.

The collected material on the electrodes is removed by rapping or vibrating the collecting
electrodes either continuously or at a predetermined interval. Cleaning a precipitator can
usually be done without interrupting the airflow.
The four main components of all electrostatic precipitators are:

1. Power supply unit, to provide high-voltage DC power

2. Ionizing section, to impart a charge to particulates in the gas stream

3. A means of removing the collected particulates

4. A housing to enclose the precipitator zone

The following factors affect the efficiency of electrostatic precipitators:

Larger collection-surface areas and lower gas-flow rates increase efficiency because of the
increased time available for electrical activity to treat the dust particles.

An increase in the dust-particle migration velocity to the collecting electrodes increases

efficiency. The migration velocity can be increased by:

Decreasing the gas viscosity

Increasing the gas temperature

Increasing the voltage field

Types of precipitators

There are two main types of precipitators:

1. High-voltage, single-stage - Single-stage precipitators combine an ionization and a

collection step. They are commonly referred to as Cottrell precipitators.

2. Low-voltage, two-stage - Two-stage precipitators use a similar principle; however, the

ionizing section is followed by collection plates.

Described below is the high-voltage, single-stage precipitator, which is widely used in minerals
processing operations. The low-voltage, two-stage precipitator is generally used for filtration in
air-conditioning systems.
Plate precipitators

The majority of electrostatic precipitators installed are the plate type. Particles are collected on
flat, parallel surfaces that are 8 to 12 in. (20 to 30 cm) apart, with a series of discharge
electrodes spaced along the centerline of two adjacent plates. The contaminated gases pass
through the passage between the plates, and the particles become charged and adhere to the
collection plates. Collected particles are usually removed by rapping the plates and deposited in
bins or hoppers at the base of the precipitator.

Tubular precipitators

Tubular precipitators consist of cylindrical collection electrodes with discharge electrodes

located on the axis of the cylinder. The contaminated gases flow around the discharge electrode
and up through the inside of the cylinders. The charged particles are collected on the grounded
walls of the cylinder. The collected dust is removed from the bottom of the cylinder.

Tubular precipitators are often used for mist or fog collection or for adhesive, sticky, radioactive,
or extremely toxic materials.

Unit collectors

Unlike central collectors, unit collectors control contamination at its source. They are small and
self-contained, consisting of a fan and some form of dust collector. They are suitable for
isolated, portable, or frequently moved dust-producing operations, such as bins and silos or
remote belt-conveyor transfer points. Advantages of unit collectors include small space
requirements, the return of collected dust to main material flow, and low initial cost. However,
their dust-holding and storage capacities, servicing facilities, and maintenance periods have
been sacrificed.

Selecting a dust collector

Dust collectors vary widely in design, operation, effectiveness, space requirements,

construction, and capital, operating, and maintenance costs. Each type has advantages and
disadvantages. However, the selection of a dust collector should be based on the following
general factors:
Dust concentration and particle size - For minerals processing operations, the dust
concentration can range from 0.1 to 5.0 grains (0.32 g) of dust per cubic feet of air (0.23 to
11.44 grams per standard cubic meter), and the particle size can vary from 0.5 to 100
micrometres (m) in diameter.

Degree of dust collection required - The degree of dust collection required depends on its
potential as a health hazard or public nuisance, the plant location, the allowable emission rate,
the nature of the dust, its salvage value, and so forth. The selection of a collector should be
based on the efficiency required and should consider the need for high-efficiency, high-cost
equipment, such as electrostatic precipitators; high-efficiency, moderate-cost equipment, such
as baghouses or wet scrubbers; or lower cost, primary units, such as dry centrifugal collectors.

Characteristics of airstream - The characteristics of the airstream can have a significant impact
on collector selection. For example, cotton fabric filters cannot be used where air temperatures
exceed 180 F (82 C). Also, condensation of steam or water vapor can blind bags. Various
chemicals can attack fabric or metal and cause corrosion in wet scrubbers.

Characteristics of dust - Moderate to heavy concentrations of many dusts (such as dust from
silica sand or metal ores) can be abrasive to dry centrifugal collectors. Hygroscopic material can
blind bag collectors. Sticky material can adhere to collector elements and plug passages. Some
particle sizes and shapes may rule out certain types of fabric collectors. The combustible nature
of many fine materials rules out the use of electrostatic precipitators.

Methods of disposal - Methods of dust removal and disposal vary with the material, plant
process, volume, and type of collector used. Collectors can unload continuously or in batches.
Dry materials can create secondary dust problems during unloading and disposal that do not
occur with wet collectors. Disposal of wet slurry or sludge can be an additional material-
handling problem; sewer or water pollution problems can result if wastewater is not treated
properly.
Industrial oven
Industrial ovens are heated chambers used for a variety of industrial applications, including
drying, curing, or baking components, parts or final products. Industrial ovens can be used for
large or small volume applications, in batches or continuously with a conveyor line, and a
variety of temperature ranges, sizes and configurations.
Such ovens are used in many different applications, including chemical processing, food
production, and even in the electronics industry, where circuit boards are run through a
conveyor oven to attach surface mount components.

Some common types of industrial ovens include:

Curing ovens Designed to cause a chemical reaction in a substance once a specific

temperature is reached. Powder coating is one common curing oven use.

Drying ovens Designed to remove moisture. Typical applications are pre-treating and
painting. Such ovens are also sometimes known as kilns, though they do not reach the
same high temperatures as are used in ceramic kilns.

Baking ovens Combines the function of curing and drying ovens.

Reflow Ovens A reflow oven is a machine used primarily for reflow soldering of surface
mount electronic components to printed circuit boards (PCB).

A graphical example of a convection reflow oven.

The oven contains multiple zones, which can be individually controlled for temperature.
Generally there are several heating zones followed by one or more cooling zones. The PCB
moves through the oven on a conveyor belt, and is therefore subjected to a controlled time-
temperature profile.

Batch ovens Also called cabinet or Walk-in/Truck-in ovens, batch ovens allow for curing, drying
or baking in small batches using wheeled racks, carts or trucks. Ovens such as this are often
found in large-volume bakeries in places such as supermarkets.

Conveyor or Continuous Ovens Typically part of an automated conveyor processing line,

conveyor ovens allow for higher volume processing.

Clean room ovens Designed for applications requiring a cleanroom, such as a semiconductor
manufacturing or biotechnology processes.
Powder coating

Powder Coating is a type of coating that is applied as a free-flowing, dry powder. The main
difference between a conventional liquid paint and a powder coating is that the powder coating
does not require a solvent to keep the binder and filler parts in a liquid suspension form. The
coating is typically applied electrostatically and is then cured under heat to allow it to flow and
form a "skin". The powder may be a thermoplastic or a thermoset polymer. It is usually used to
create a hard finish that is tougher than conventional paint. Powder coating is mainly used for
coating of metals, such as household appliances, aluminum extrusions, drum hardware, and
automobile and bicycle parts. Newer technologies allow other materials, such as MDF (medium-
density fibreboard), to be powder coated using different methods.

Properties of powder coating

Because powder coating does not have a liquid carrier, it can produce thicker coatings than
conventional liquid coatings without running or sagging, and powder coating produces minimal
appearance differences between horizontally coated surfaces and vertically coated surfaces.
Because no carrier fluid evaporates away, the coating process emits few volatile organic
compounds (VOC). Finally, several powder colors can be applied before curing them all together,
allowing color blending and bleed special effects in a single layer.
While it is relatively easy to apply thick coatings which cure to smooth, texture-free coating, it is
not as easy to apply smooth thin films. As the film thickness is reduced, the film becomes more
and more orange peeled in texture due to the particle size and glass transition temperature (Tg)
of the powder.

Most powder coatings have a particle size in the range of 2 to 50 (Microns), a softening
temperature Tg around 80 C, a melting temperature around 150 C, and are cured at around
200 C. for minimum 10 minutes to 15 minutes (exact temperatures and times may depend on
the thickness of the item being coated). [1] For such powder coatings, film build-ups of greater
than 50 (Microns) may be required to obtain an acceptably smooth film. The surface texture
which is considered desirable or acceptable depends on the end product. Many manufacturers
actually prefer to have a certain degree of orange peel since it helps to hide metal defects that
have occurred during manufacture, and the resulting coating is less prone to showing
fingerprints.

Aluminium extrusions being powder coated

There are very specialized operations where powder coatings of less than 30 micrometres or
with a Tg below 40 C are used in order to produce smooth thin films. One variation of the dry
powder coating process, the Powder Slurry process, combines the advantages of powder
coatings and liquid coatings by dispersing very fine powders of 15 micrometre particle size into
water, which then allows very smooth, low film thickness coatings to be produced.

For garage-scale jobs, small "rattle can" spray paint is less expensive and complex than powder
coating. At the professional scale, the capital expense and time required for a powder coat gun,
booth and oven are similar to a spray gun system. Powder coatings have a major advantage in
that the overspray can be recycled. However, if multiple colors are being sprayed in a single
spray booth, this may limit the ability to recycle the overspray.

Advantages of powder coating instead of other coatings

Powder coatings contain no solvents and release little or no amount of Volatile Organic
Compounds (VOC) into the atmosphere. Thus, there is no longer a need for finishers to buy
 Before and after powder coating

1. . Companies can comply more easily and economically with the regulations of the U.S.
Environmental Protection Agency.[2]

2. Powder coatings can produce much thicker coatings than conventional liquid coatings
without running or sagging.

3. Powder coated items generally have fewer appearance differences between horizontally
coated surfaces and vertically coated surfaces than liquid coated items.

4. The wide range of specialty effects are easily accomplished using powder coatings that
would be impossible to achieve with other coating processes.[3]

5. Curing time is significantly faster with powder coating as compared to liquid coating.
Types of powder coating

There are two main categories of powder coating:

thmosets and thermoplastics. The thermosetting variety incorporates a cross-linker into the
formulation. When the powder is baked, it reacts with other chemical groups in the powder to
polymerize, improving the performance properties. The thermoplastic variety does not undergo
any additional actions during the baking process as it flows to form the final coating.

The most common polymers used are polyester, polyurethane, polyester-epoxy (known as
hybrid), straight epoxy (fusion bonded epoxy) and acrylics.

The powder coating process

The powder coating process involves three basic steps:

1. Part preparation or the pre-treatment

2. The powder application

3. Curing

Part preparation processes and equipment

Removal of oil, dirt, lubrication greases, metal oxides, welding scale etc. is essential prior to the
powder coating process. It can be done by a variety of chemical and mechanical methods. The
selection of the method depends on the size and the material of the part to be powder coated,
the type of impurities to be removed and the performance requirement of the finished product.

Chemical pre-treatments involve the use of phosphates or chromates in submersion or spray

application. These often occur in multiple stages and consist of degreasing, etching, de-
smutting, various rinses and the final phosphating or chromating of the substrate. The pre-
treatment process both cleans and improves bonding of the powder to the metal. Recent
additional processes have been developed that avoid the use of chromates, as these can be
toxic to the environment. Titanium zirconium and silanes offer similar performance against
corrosion and adhesion of the powder.
In many high end applications, the part is electrocoated following the pretreatment process,
and subsequent to the powder coating application. This has been particularly useful in
automotive and other applications requiring high end performance characteristics.

Another method of preparing the surface prior to coating is known as abrasive blasting or
sandblasting and shot blasting. Blast media and blasting abrasives are used to provide surface
texturing and preparation, etching, finishing, and degreasing for products made of wood,
plastic, or glass. The most important properties to consider are chemical composition and
density; particle shape and size; and impact resistance.

Silicon carbide grit blast medium is brittle, sharp, and suitable for grinding metals and low-
tensile strength, non-metallic materials. Plastic media blast equipment uses plastic abrasives
that are sensitive to substrates such as aluminum, but still suitable for de-coating and surface
finishing. Sand blast medium uses high-purity crystals that have low-metal content. Glass bead
blast medium contains glass beads of various sizes.

Cast steel shot or steel grit is used to clean and prepare the surface before coating. Shot blasting
recycles the media and is environmentally friendly. This method of preparation is highly efficient
on steel parts such as I-beams, angles, pipes, tubes and large fabricated pieces.

Different powder coating applications can require alternative methods of preparation such as
abrasive blasting prior to coating. The online consumer market typically offers media blasting
services coupled with their coating services at additional costs.

Powder application processes

Example of powder coating spray guns

The most common way of applying the powder coating to metal objects is to spray the powder
using an electrostatic gun, or corona gun. The gun imparts a positive electric charge to the
powder, which is then sprayed towards the grounded object by mechanical or compressed air
spraying and then accelerated toward the workpiece by the powerful electrostatic charge. There
are a wide variety of spray nozzles available for use in electrostatic coating. The type of nozzle
used will depend on the shape of the workpiece to be painted and the consistency of the paint.
The object is then heated, and the powder melts into a uniform film, and is then cooled to form
a hard coating. It is also common to heat the metal first and then spray the powder onto the hot
substrate. Preheating can help to achieve a more uniform finish but can also create other
problems, such as runs caused by excess powder. See the article "Fusion Bonded Epoxy
Coatings"

Another type of gun is called a tribo gun, which charges the powder by (triboelectric) friction. In
this case, the powder picks up a positive charge while rubbing along the wall of a Teflon tube
inside the barrel of the gun. These charged powder particles then adhere to the grounded
substrate. Using a tribo gun requires a different formulation of powder than the more common
corona guns. Tribo guns are not subject to some of the problems associated with corona guns,
however, such as back ionization and the Faraday cage effect.
Powder can also be applied using specifically adapted electrostatic discs.

Another method of applying powder coating, called the fluidized bed method, is by heating the
substrate and then dipping it into an aerated, powder-filled bed. The powder sticks and melts to
the hot object. Further heating is usually required to finish curing the coating. This method is
generally used when the desired thickness of coating is to exceed 300 micrometres. This is how
most dishwasher racks are coated.

Electrostatic fluidized bed coating

Electrostatic fluidized bed application uses the same fluidizing technique as the conventional
fluidized bed dip process but with much less powder depth in the bed. An electrostatic charging
medium is placed inside the bed so that the powder material becomes charged as the fluidizing
air lifts it up. Charged particles of powder move upward and form a cloud of charged powder
above the fluid bed. When a grounded part is passed through the charged cloud the particles
will be attracted to its surface. The parts are not preheated as they are for the conventional
fluidized bed dip process.

Electrostatic magnetic brush (EMB) coating

A coating method for flat materials that applies powder with a roller, enabling relatively high
speeds and accurate layer thickness between 5 and 100 micrometres. The base for this process
is conventional copier technology. It is currently in use in some coating applications and looks
promising for commercial powder coating on flat substrates (steel, aluminium, MDF, paper,
board) as well as in sheet to sheet and/or roll to roll processes. This process can potentially be
integrated in an existing coating line.

Curing

When a thermoset powder is exposed to elevated temperature, it begins to melt, flows out, and
then chemically reacts to form a higher molecular weight polymer in a network-like structure.
This cure process, called crosslinking, requires a certain temperature for a certain length of time
in order to reach full cure and establish the full film properties for which the material was
designed. Normally the powders cure at 200 C (390 F) for 10 minutes. The curing schedule
could vary according to the manufacturer's specifications. The application of energy to the
product to be cured can be accomplished by convection cure ovens, infrared cure ovens, or by
laser curing process. The latter demonstrates significant reduction of curing time.

Removing powder coating

Methylene chloride and acetone are generally effective at removing powder coating. Most other
organic solvents (thinners, etc.) are completely ineffective. Recently the suspected human
carcinogen methylene chloride is being replaced by benzyl alcohol with great success. Powder
coating can also be removed with abrasive blasting. 98% sulfuric acid commercial grade also
removes powder coating film.[citation needed] Certain low grade powder coats can be removed
with steel wool, though this might be a more labor-intensive process than desired.

Powder coating can also be removed by a burning off process, in which parts are put into a large
high-temperature oven with temperatures typically reaching an air temperature of 300 - 450 C.
The process takes about four hours and requires the parts to be cleaned completely and re-
powder coated. Parts made with a thinner-gauge material need to be burned off at a lower
temperature to prevent the material from warping.
Diferent elements of powder coating gun
Rotary screw compressors
 rotary-screw compressor is a type of gas compressor that uses a rotary-type positive-
displacement mechanism. They are commonly used to replace piston compressors where large
volumes of high-pressure air are needed, either for large industrial applications or to operate
high-power air tools such as jackhammers.

The gas compression process of a rotary screw is a continuous sweeping motion, so there is very
little pulsation or surging of flow, as occurs with piston compressors.

screw compressors use two meshing helical screws, known as rotors, to compress the gas. In a
dry-running rotary-screw compressor, timing gears ensure that the male and female rotors
maintain precise alignment. In an oil-flooded rotary-screw compressor, lubricating oil bridges
the space between the rotors, both providing a hydraulic seal and transferring mechanical
energy between the driving and driven rotor. Gas enters at the suction side and moves through
the threads as the screws rotate. The meshing rotors force the gas through the compressor, and
the gas exits at the end of the screws.

The effectiveness of this mechanism is dependent on precisely fitting clearances between the
helical rotors and between the rotors and the chamber for sealing of the compression cavities.
However, some leakage is inevitable, and high rotational speeds must be used to minimize the
ratio of leakage flow rate over effective flow rate.

In contrast to Roots blowers, screw compressors are made with different profiles on the two
rotors: the male rotor has convex lobes which mesh with the concave cavities of the female
rotor. Usually the male rotor has fewer lobes than the female rotor, so that it rotates faster.
Originally, screw compressors were made with symmetrical rotor cavity profiles, but modern
versions use asymmetrical rotors, with the exact rotor designs being the subject of patents.[2]

Size

Rotary-screw compressors tend to be compact and smoothly running with limited vibration,
thus not requiring spring suspension. Many rotary-screw compressors are, however, mounted
using rubber vibration-isolating mounts to absorb high-frequency vibrations, especially in
rotary-screw compressors that operate at high rotational speeds. Rotary-screw compressors are
produced in sizes that range in pumping speeds from 10 cubic feet per minute to several
thousand CFM. Rotary-screw compressors are typically used in applications requiring more
airflow than is produced by small reciprocating compressors but less than is produced by
centrifugal compressors.

Applications

Typically, they are used to supply compressed air for general industrial applications. Trailer-
mounted diesel-powered units are often seen at construction sites and are used to power air-
operated construction machinery.

Oil-free

In an oil-free compressor, the air is compressed entirely through the action of the screws,
without the assistance of an oil seal. They usually have lower maximal discharge pressure
capability as a result. However, multi-stage oil-free compressors, where the air is compressed by
several sets of screws, can achieve pressures of over 150 psi (10 atm) and output volume of
over 2,000 cubic feet per minute (57 m3/min).

Oil-free compressors are used in applications where entrained oil carry-over is not acceptable,
such as medical research and semiconductor manufacturing. However, this does not preclude
the need for filtration, as hydrocarbons and other contaminants ingested from the ambient air
must also be removed prior to the point of use. Subsequently, air treatment identical to that
used for an oil-flooded screw compressor is frequently still required to ensure a given quality of
compressed air.

The twin-screw type supercharger is a positive displacement type device that operates by
pushing air through a pair of meshing close-tolerance screws similar to a set of worm gears.
Twin-screw superchargers are also known as Lysholm superchargers (or compressors) after their
inventor, Alf Lysholm.[6] Each rotor is radially symmetrical, but laterally asymmetric. By
comparison, conventional "Roots" type blowers have either identical rotors (with straight
rotors) or mirror-image rotors (with helixed rotors). The Whipple-manufactured male rotor has
three lobes, the female five lobes. The Kenne-Bell male rotor has four lobes, the female six
lobes. Females in some earlier designs had four. By comparison, Roots blowers always have the
same number of lobes on both rotors, typically 2, 3 or 4. The working area is the inter-lobe
volume between the male and female rotors. Its larger at the intake end, and decreases along
the length of the rotors until the exhaust port. This change in volume is the compression. The
intake charge is drawn in at the end of the rotors in the large clearance between the male and
female lobes. At the intake end the male lobe is much smaller than its female counterpart, but
the relative sizes reverse proportions along the lengths of both rotors (the male becomes larger
and the female smaller) until (tangential to the discharge port) the clearance space between
each pair of lobes is much smaller. This reduction in volume causes compression of the charge
before being presented to the output manifold.

Comparative advantages

The rotary screw compressor has low leakage levels and low parasitic losses vs. Roots type. The
supercharger is typically driven directly from the engine's crankshaft via a belt or gear drive.
Unlike the Roots type supercharger, the twin-screw exhibits internal compression which is the
ability of the device to compress air within the housing as it is moved through the device
instead of relying upon resistance to flow downstream of the discharge to establish an increase
of pressure.[7]

The requirement of high-precision computer-controlled manufacturing techniques makes the

screw type supercharger a more expensive alternative to other forms of available forced
induction. With later technology, manufacturing cost has been lowered while performance
increased.

All supercharger types benefit from the use of an intercooler to reduce heat produced during
pumping and compression.

A clear example of the technology applied by the twin-screw in companies like Ford, Mazda,
Mercedes and Mercury Marine can also demonstrate the effectiveness of the twin screw. While
some centrifugal superchargers are consistent and reliable, they typically do not produce full
boost until near peak engine rpm, while positive displacement superchargers such as Roots type
superchargers .